The Digital Photography School blog has an instructive tutorial on using layers for those just getting their feet wet inside Photoshop, or other high-end image editors like the open-source GIMP. Those jumping in will learn how to make transparent layers, use masked layers to roll back effects, and duplicate layers for sharpening and other effects. Hit the link for a quick schooling, including links to related lessons, or offer up your own tutorials and advice in the comments.